1. Field of the Invention
The present invention relates to a zoom lens having an image stabilizing function for optically correcting blur of a taken image due to shake or vibration, which is suitable for an image pickup apparatus such as a TV camera, a digital camera or a video camera.
2. Description of the Related Art
Conventionally, an image pickup optical system which is used for a TV camera, a video camera or the like is required to be a zoom lens having a large diameter, a high zoom ratio and high optical performance.
In addition to this, operability and mobility are important for a broadcast TV camera in particular. In order to respond to the requirements, a ⅔ inch or ½ inch small solid-state image pickup element (CCD) is used as an image pickup device in many cases. This CCD has substantially uniform resolving power over the entire image pickup range. Therefore, the zoom lens using the CCD is required to have substantially uniform resolving power from the center to periphery of a screen. For example, it is required that various aberrations such as coma, astigmatism and distortion are corrected appropriately so that the entire screen has high optical performance. Further, the zoom lens is required to have a large diameter, a wide angle of field, and a high zoom ratio with small size and light weight. In addition, the zoom lens is required to have a long back focus because a color separation optical system and various filters are disposed on the object side of the image pickup unit. Other than that, in a zoom lens having a high zoom ratio with the zoom range of long focal length, it is important to reduce image blur due to vibration or shake. Therefore, the zoom lens is demanded to have the image stabilizing function for reducing blur of an image.
As a zoom lens having the image stabilizing function, there is known a zoom lens which corrects blur of a taken image by decentering a sub lens unit constituting the zoom lens in the direction perpendicular to the optical axis (U.S. Pat. No. 6,560,016, and Japanese Patent Application Laid-Open No. 2001-100089).
The zoom lens having the image stabilizing function has an advantage in that a special optical system for image stabilizing is not required. However, more space is necessary for moving the lens unit, and large decentering aberration is generated in the image stabilizing. Therefore, an optical arrangement of the image stabilizing lens unit and the lens unit disposed on the image side of the image stabilizing lens unit is devised so as to correct the decentering aberration generated in the image stabilizing.
In U.S. Pat. No. 6,560,016 and Japanese Patent Application Laid-Open No. 2001-100089, the zoom lens is constituted by a first lens unit that is not moved for zooming, a second lens unit as a magnification-varying unit, a third lens unit for correcting image plane variation due to magnification-varying and a fourth lens unit that is not moved for zooming. Further, there is disclosed a zoom lens including an image stabilizing lens unit disposed at the most object side in the fourth lens unit. In addition, in U.S. Pat. No. 6,560,016 and Japanese Patent Application Laid-Open No. 2001-100089, a movement amount in the direction perpendicular to the optical axis when the image stabilizing lens unit is decentered is increased, to thereby enhance the effect of correcting blur of the image. Thus, the drive mechanism is downsized, and deterioration of image quality due to the decentering is reduced. Matsui (the 23rd meeting proceeding of the Japan Society of Applied Physics, 1962) discloses a method of aberration correction from a standpoint of aberration theory on generation of decentering aberration when a sub system in the optical system is decentered in the direction perpendicular to the optical axis.
In the optical system in which the image stabilizing is performed by decentering the image stabilizing lens unit in the direction perpendicular to the optical axis, the image stabilizing lens unit is shifted also in the optical axis direction not a little. As a result, there is a problem that a focus variation occurs. In particular, in a broadcast zoom lens, using a built-in extender or the like, a magnification-varying range (zooming range) is shifted to the telephoto side or to the wide-angle side by inserting or removing the unit into or from the optical axis. In this case, focus variation caused by image stabilizing drive may be increased or decreased by the unit switching, which becomes a large practical problem.
In addition, in order to improve trackability of the correction to blur of the image and to reduce a load on the drive system, it is required that the image stabilizing lens unit is compact and lightweight. However, if particularly high optical performance is demanded also in decentering as in the case of the broadcast zoom lens having a high zoom ratio, it is so difficult to achieve a compact and lightweight image stabilizing lens unit while maintaining good optical performance in image stabilizing.
It is important for the zoom lens having the image stabilizing function and high zoom ratio to reduce the focus variation due to the image stabilizing drive and to appropriately correct aberrations due to decentering, in particular, decentering coma and decentering lateral chromatic aberration.